1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same including the switching elements having the MOS structure, and more specifically to a semiconductor device and a method for manufacturing the same that can improve reliability.
2. Background Art
In conventional vertical power MOSFETs, the gate electrodes are formed of poly-silicon, which has a poor conductivity. Therefore, the supply of a potential to the gate electrodes of respective unit cells is facilitated by forming a gate wiring consisting of a metal film composed of Al or the alloy thereof, or copper on the periphery of the chip to make the speed of switching higher (e.g., refer to Patent Documents 1 and 2). In the semiconductor under the gate wiring or the gate pad, a p-type well is formed for helping the elongation of the depletion layer and preventing the deterioration of pressure resistance.
A semiconductor device wherein minute diodes are linearly placed on the periphery of the cell region where unit cells are formed (including the gate pad portion) has been proposed (e.g., refer to FIGS. 1 and 2 of Patent Document 1). This diode can absorb holes from the p-type well to the n-type drain layer on the time of forward bias when the MOSFET is switched (turn-off) from the ON-state (forward bias) to the OFF-state (reverse bias), and the parasitic transistor can be prevented from turning ON (e.g., refer to FIG. 3 of Patent Document 1).
When the MOSFET is turned OFF, the voltage of the drain electrode (drain voltage) is rapidly elevated from 0 to several hundred volts. For this reason, the displacement current is flowed into the p-type well via the parasitic capacitor present between the p-type well and the n-type drain layer. This is the same in the p-type well of the MOSFET, the p-type well of a diode, or the p-type well below that gate wiring.
The p-type well is electrically connected to a field plate via a contact hole, and the field plate is electrically connected to a source electrode. Therefore, the displacement current flowed into the p-type well below the gate wiring flows into the source electrode via the contact hole and the field plate.    Patent Document 1: Japanese Patent Application Laid Open No. 5-198816    Patent Document 2: Japanese Patent Application Laid Open No. 2006-19608